Segmental retaining walls commonly comprise courses of modular units (blocks). The blocks are typically made of concrete. The blocks are typically dry-stacked (no mortar or grout is used), and often include one or more features adapted to properly locate adjacent blocks and/or courses with respect to one another, and to provide resistance to shear forces from course to course. The weight of the blocks is typically in the range of ten to one hundred fifty pounds per unit. Segmental retaining walls commonly are used for architectural and site development applications. Such walls are subjected to high loads exerted by the soil behind the walls. These loads are affected by, among other things, the character of the soil, the presence of water, temperature and shrinkage effects, and seismic loads. To handle the loads, segmental retaining wall systems often comprise one or more layers of soil reinforcement material extending from between the courses of blocks back into the soil behind the blocks. The reinforcement material is typically in the form of a geogrid or a geofabric. Geogrids often are configured in a lattice arrangement and are constructed of polymer fibers or processed plastic sheet material (punched and stretched, such as described, for example, in U.S. Pat. No. 4,374,798), while reinforcement fabrics are constructed of woven, nonwoven, or knitted polymer fibers or plastics. These reinforcement members typically extend rearwardly from the wall and into the soil to stabilize the soil against movement and thereby create a more stable soil mass which results in a more structurally secure retaining wall. In other instances, the reinforcement members comprise tie-back rods that are secured to the wall and which similarly extend back into the soil.
Although several different forms of reinforcement members have been developed, opportunities for improvement remain with respect to attachment of the reinforcement members to the facing blocks in the retaining wall systems. As a general proposition, the more efficient the block/grid connection, the fewer the layers of grid that should be required in the wall system. The cost of reinforcing grid can be a significant portion of the cost of the wall system, so highly efficient block/grid connections are desirable.
Many segmental retaining wall systems rely primarily upon frictional forces to hold the reinforcement material between adjacent courses of block. These systems may also include locating pins or integral locator/shear resistance features that enhance the block/grid connection to varying degrees. Examples of such systems include those described in U.S. Pat. Nos. 4,914,876, 5,709,062, and 5,827,015. These systems cannot take advantage of the full tensile strength of the common reinforcement materials, however, because the block/grid holding forces that can be generated in these systems is typically less than the tensile forces that the reinforcing materials themselves can withstand.
One of the many advantages of segmental retaining wall systems over other types of retaining walls is their flexibility. They do not generally require elaborate foundations, and they can perform well in situations where there is differential settling of the earth, or frost heaving, for example, occurs. Even so, these types of conditions might result in differentials in the block/grid connections across the wall in systems that rely primarily on fricitional connection of blocks to grid.
In an effort to improve the grid/block connection efficiency, several current retaining wall systems have been developed that mechanically connect the reinforcement members to the blocks. In several such systems, rake shaped connector bars are positioned transversely in the center of the contact area between adjacent stacked blocks with the prongs of the connector bars extending through elongated apertures provided in the geogrid to retain it in place. Examples of this type of system are shown in U.S. Pat. Nos. 5,607,262 (FIGS. 1-7), 5,417,523, and 5,540,525. These systems are only effective if the geogrid used is of a construction such that the cross-members that engage the prongs of the connector will resist the tensile forces exerted on the grid by the soil. There are only a few such grids currently available and, thus, the wall builder or contractor has to select geogrid products from a limited number of reinforcement member manufacturers when such an attachment system is used. These systems also rely upon the prongs of the rake connectors being in register with the apertures in the grid material and in contact with the grid cross members. If the connector prongs do not line up with the grid apertures, installation becomes a problem. Variability in the grid manufacturing process means that the apertures in this type of grid frequently are not perfectly regular. A solution to this problem has been to use short connector rakes that only engage several grid apertures, rather than long connectors that engage all of the apertures in a row across the grid layer. This solution eases installation problems, but would appear to make the connection mechanism less efficient, with the consequence that the full strength of the grid cannot be taken advantage of in the design of the wall system. These devices are subject to the same criticisms as the pure friction connector systems.
A third type of connector system uses a channel that, in cross-section, has a relatively large inner portion and a very narrow opening out of that portion. The grid is provided with a bead or equivalent enlargement along its leading edge. The grid is then threaded into the channel from the side, so that the grid layer extends out through the narrow channel opening, but the bead is captured in the larger inner portion. An example of this type of connection is shown in FIGS. 9-10 of U.S. Pat. No. 5,607,262. While this system overcomes differential settling concerns, it is very difficult to use in the field, and relies upon special grid configurations.
A modification of the third type of connector system described above is one in which the channel into which the bead fits is formed by a combination of the lower and adjacent upper block, so that the enlarged/beaded end of the grid can simply be laid in the partial channel of the lower blocks, and will be captured when the upper blocks are laid. This system simplifies installation, but does not resolve the aforementioned performance concerns. In a variation of this system, the end of a panel of geogrid material is wrapped around a bar, which is then placed in a hollowed-out portion of the facing unit which is provided with an integral stop to resist pullout of the bar. Rather than being held in place by the next above facing unit, the wrapped bar is then weighted down with earth or gravel fill dumped on top of it in the hollowed out portion of the facing unit. This system is shown in U.S. Pat. No. 5,066,169. Not only is the facing unit of this system extremely complex and difficult to make, but the installation process is difficult and requires the use of very narrow panels of grid material.
From the above, it can be appreciated that it would be desirable to have a segmental retaining wall system comprising a facing block of a relatively simple shape to facilitate high speed mass production, and wherein the block can be mechanically connected to the reinforcement material in a fashion that is highly efficient, so that a higher percentage of the full design strength of the reinforcement can be taken advantage of, wherein the system can be used with a wide variety of the commonly available geogrids and fabrics, wherein the grid/block connection mechanism is secure even in differential settling conditions, and wherein the system is easy to work with in the field during installation.